A wind turbine is an energy converting system which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind is applied to wind turbine blades of the wind turbine to rotate a rotor. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. Because wind speed fluctuates, the force applied to the wind blades and hence the rotational speed of the rotor can vary. Power grids however require a constant frequency electrical power to be provided by the wind turbine.
One type of wind turbine that provides constant frequency electrical power is a fixed-speed wind turbine. This type of wind turbine requires a generator rotor that rotates at a constant speed. A disadvantage of such fixed-speed wind turbine is that it does not harness all of the wind's energy at high speeds and must be disabled at low wind speeds. Another type of wind turbine is a variable speed wind turbine. This type of wind turbine allows the generator to rotate at variable speeds to accommodate for fluctuating wind speeds. By varying the rotating speed of the generator rotor, energy conversion can be optimized over a broader range of wind speeds.
A variable speed wind turbine usually includes a power converter having a generator side converter coupled to a grid side converter via a direct current (DC) link. The generator side converter regulates the power of the generator. This power passes through the DC-link, fully or partially, and is eventually fed to the grid through the grid side converter.
Conventionally, when there is a fault in the grid, the wind turbine (or wind farm comprising wind turbines) is usually disconnected from the grid to protect the electrical components and mechanical parts of the turbine due to relaxed grid code requirements. Subsequently, when the fault is cleared, the wind turbine is re-connected to the grid again to supply power thereto.
With increasing penetration of wind power generation, the disconnection of wind turbines or wind farms from the grid during grid faults is no longer acceptable by grid operators. This is because the disconnection of a wind turbine/farm under grid fault condition leads to instability of the power grid. Grid operators in many countries now require wind farm operators to comply with certain grid requirements specified in grid codes before they are allowed to connect to the grid. Grid requirements vary in different countries, but they have a common aim of permitting the development, maintenance and operation of a coordinated, reliable and economical transmission or distribution system. For example, grid codes typically require that wind turbines should be able to ride-through a fault causing the voltage at a Point of Common Coupling at a wind farm to decrease to, for example 0.2 pu with duration of 0.5 seconds.
U.S. Pat. No. 6,921,985 discloses the use of a shunt circuit or a crowbar circuit to shunt the sudden surge of current during a grid fault, such as a voltage dip. The crowbar circuit is coupled to a point between the generator side converter and the generator rotor. When a sudden increase in the current at the generator side converter is detected, the crowbar circuit is activated, and the current is drained from the generator side converter by the crowbar circuit.
U.S. Pat. No. 7,411,309 also discloses the use of a crowbar circuit during voltage transients at the grid. The crowbar circuit is coupled to the DC link between the generator side converter and the grid side converter. When the DC link voltage exceeds a predetermined value (due to grid fault), the crowbar circuit is activated to drain the current from the generator side converter, hence lowering the DC link voltage.
In both the prior art documents, the shunt circuits use resistors to drain any undesired surge of current or dissipate any excess power generated due to grid fault. As the power output of the wind turbines are large, the shunt circuit need to use high mega joules resistors in order to dissipate excess power from the generator. In addition, the heat generated by the power dissipating resistors needs to be handled. This may require cooling for the power dissipating resistors.
It is thus an object of the invention to provide an improved energy handling circuits for managing excess power from the wind turbine without the disadvantages mentioned above.